1. Field of the Invention
The present invention relates to a power generation portion of a fuel cell.
2. Description of the Related Art
A polymer electrolyte fuel cell basically includes a polymer electrolyte membrane having proton conductivity, and a pair of catalyst layers and electrodes provided on both surfaces of the polymer electrolyte membrane.
In general, each of the catalyst layers includes a catalyst made of platinum or a metal belonging to the platinum group. A gas diffusion layer is provided on an outer surface of the catalyst layer, for supplying gas and collecting current.
An assembly in which the polymer electrolyte membrane and the catalyst layers are integrated into one is referred to as a membrane electrode assembly (MEA) having a structure in which a fuel (hydrogen) is supplied to one of electrodes and an oxidizer (oxygen) is supplied to another electrode to conduct power generation. An electric power is taken out from the electrodes on both sides.
A theoretical voltage of a pair of membrane electrode assemblies is about 1.23 V, and in a normal operation condition, the pair of membrane electrode assemblies are driven by about 0.7 V in many cases.
Accordingly, in a case where a higher activation voltage is required, plural fuel cell units are laminated and arranged electrically in series to be used. A laminate structure of this type is called a fuel cell stack.
In the stack, normally, an oxidizer flow path and a fuel flow path are isolated from each other by a member called a separator.
Each separator of a plate type is formed with concave portions and convex portions. The concave portions facing the membrane electrode assembly constitute gas paths and the convex portions constitute current collecting portions.
For the fuel cell used for portable electronic devices, it is required that auxiliary devices etc. such as a fan and a blower be omitted in order to achieve downsizing and higher output.
Oxygen serving as the oxidizer is desirably supplied by natural diffusion of air. Further, in a laminate-type stack structure, the air is taken in only through a side surface of the stack.
Accordingly, in order to sufficiently supply the air, it is necessary to increase a thickness of the separator, thereby increasing an air take-in area.
On the other hand, in order to achieve downsizing and higher output of the stack, it is necessary that the thickness of the separator be reduced as much as possible to attain high density mounting.
With the above-mentioned structure, a surface of the separator on a side of the membrane electrode assembly has a concave-convex shape, and the air take-in area to each of fuel cell units is a sum of sectional areas of the concave portions. The separator also has a function of collecting current generated in the fuel cell unit through a contact portion and of allowing the current to flow to the adjacent fuel cell unit or to a take-out electrode to the outside. It is necessary that an area occupied by the convex portions be large to a certain degree. A balance between the areas of the concave portions and the convex portions is appropriately set depending on a value of a current to be taken out. The areas of the concave portions and the convex portions have a trade-off relationship with each other. That is, it is difficult to ensure both sufficient taking-in of the air and a sufficient contact area.
Regarding this, Japanese Patent Application Laid-Open No. 2004-146265 proposes a fuel cell having a structure in which an air take-in area can be made larger than that of the separator having the concave-convex shape and nonuniformity in current collection is relieved. For those fuel cells, there is employed a fuel cell stack having a structure in which plural disk-shaped fuel cell units are laminated and atmospheric air is used.
Further, as an air take-in mechanism on the oxidizer electrode side, there is not used the separator having grooves as the gas flow paths formed therein, and there is disposed a conductive porous member.
Specifically, from the membrane electrode assembly side, a carbon paper serving as a gas diffusion layer and a foamed metal serving as a flow path forming member are laminated in the stated order.
By the conductive porous member, two functions of the gas flow path and the current collection are achieved. Therefore, compared to the separator of the concave-convex shape, the air take-in area can be made larger, and the nonuniformity in current collection is relieved.
Further, in those fuel cells, hydrogen is supplied to the fuel electrode of each of the fuel cell units through a through hole formed in a center of the disk shape, and on an end surface of an outer periphery of the fuel electrode side thereof, there is provided a sealing material so as to prevent leakage of the hydrogen.
Further, air is supplied through an outer peripheral portion of the oxidizer electrode side of the fuel cell unit though natural diffusion, and on an end surface of an inner periphery of the oxidizer electrode side thereof, there is provided a sealing material so as to prevent the air from mixing with the hydrogen.
Further, the stack is structured by fastening a central portion of the disk by a bolt.
However, the related art example disclosed in Japanese Patent Application Laid-Open No. 2004-146265 includes a portion where sealing of the fuel is imperfect.
In the fuel cell stack, the sealing of the fuel is performed by compressing the sealing material which is an elastic body by a fastening force.
However, a mating member opposed to the sealing material with respect to the polymer electrolyte membrane is an elastic body such as a carbon cloth or a carbon paper.
Accordingly, there arises a problem in that due to compression deformation of the mating elastic body, it is difficult to apply the fastening pressure to the sealing material.
Further, with the carbon cloth or the carbon paper, due to surface roughness of the material, it is difficult to uniformly apply the fastening pressure to the sealing material.
On the other hand, when consideration is made only of the sealing and the fastening pressure is increased, there is increased a risk of occurrence of problems such as breakage of the polymer electrolyte membrane and increase in gas flow path resistance due to excessive deformation of the flow path forming member.
Accordingly, it is difficult to perfectly perform the sealing and there is a concern about leakage of the fuel.